The present invention pertains to nuclear magnetic resonance (NMR) methods and systems, and in particular to NMR spectroscopy and magnetic resonance imaging (MRI) methods and systems for improving the dynamic range of the received signal by using RF pulses that produce a spatially varying phase that is substantially quadratic.
MRI and NMR spectroscopy each involve placing a sample to be measured (e.g., a portion of the body) in a high, uniform magnetic field (i.e., a polarizing field B0 in the Z-direction). The spins of the nuclei in the sample attempt to align with the field, but precess about it in random order at their characteristic Larmor frequency. If the sample is subject to an oscillating magnetic field (usually produced by radio frequency (RF) pulses) in the X-Y plane with a frequency near the Larmor frequency, the spins are rotated or xe2x80x9ctippedxe2x80x9d in the X-Y plane to produce a net transverse magnetic moment. A signal is produced by the excited spins, and when the RF magnetic field ceases, the signal can be detected by a receiver coil arranged adjacent the sample. The receiver coil converts the received signal into an electrical signal, which can then be analyzed.
Depending on the nature of the RF magnetic field and how it is transmitted to and received from the sample, 2-dimensional (2-D) or 3-dimensional (3-D) MRI of a portion of the sample can be obtained. Such images can be used for diagnosing a variety of physical conditions (e.g., the presence of cancer cells). Alternatively, NMR spectra of the xe2x80x9cchemical shiftxe2x80x9d of a substance can be measured. Background information about NMR spectroscopy and MRI can be found in the book by S. Webb, The Physics of Medical Imaging, Institute of Physics Publishing, Ltd., 1992.
In xe2x80x9cslab selectivexe2x80x9d or xe2x80x9cvolume selectivexe2x80x9d imaging, a thick slice (xe2x80x9cslabxe2x80x9d) of the sample is excited with one or more RF pulses and several gradient pulses. The gradient pulses encode spatial information. In a typical 3D image there is a readout gradient and two phase encoding gradients. This form of MRI has the advantage that the slices are contiguous, but also has the disadvantage that the signal from the entire excited slab must be collected with each scan. These large signals can overwhelm the acquisition analog-to-digital converter (ADC). In an effort to match the range of the signal with the dynamic range of the ADC so that the smallest signals are properly digitized, commonly the signal is over-amplified and then the amplitude reduced by a variable attenuator to fill the dynamic range of the ADC.
When the analog signal is converted into a digital representation, the continuous signals are transformed into discrete steps. With very large signals, an attenuation setting that reduces the peak signal so that it does not exceed the limits of the ADC can reduce the low amplitude signals to a point that they can not be sufficiently described by the discrete steps. When this occurs, fluctuations in the least significant bits of the ADC degrade the resulting image. This same problem arises in performing NMR spectroscopy where large signals are processed. For example, in performing NMR on a section of the head, the lipid signal from the subcutaneous fat is several orders of magnitude higher than the metabolite resonances from the brain.
Various techniques have been developed in an attempt to improve the dynamic range of MRI. The techniques generally involve varying the amplitude and phase (or frequency) of the RF excitation pulse so that the amplitude of the spin-response is reduced. One such technique, for example, involves using an RF pulse with a randomized phase, as described in the paper by A. Maudsley, entitled xe2x80x9cDynamic Range Improvement in MRI Using Phase Scrambling,xe2x80x9d Journal of Magnetic Resonance, 76, 287-305 (1988). Another technique involves the use of an RF pulse with a constant amplitude and a linearly varying frequency (a chirp pulse). Related techniques have been developed for NMR spectroscopy, and are referred to as xe2x80x9cchemical shift imaging with phase-encoding RF pulses,xe2x80x9d or CSI-PP.
Many of the phase-shifting techniques developed to date for slab-selective MRI and NMR spectroscopy do indeed reduce the dynamic range of the signal. Unfortunately however, they also have the serious shortcomings that the uniformity of the passband and the sharpness of the slab-selection excitation profile, often denoted MXY (hereinafter, xe2x80x9cslab profilexe2x80x9d), are significantly reduced. This degrades the resulting MRI image or the NMR chemical-shift spectrum.
Accordingly, there is a need for systems and methods applicable to slab-selective 3-D MRI and NMR spectroscopy that reduce the peak amplitude of the received signal while also maintaining a sharp slab profile.
The present invention pertains to nuclear magnetic resonance (NMR) methods and systems, and in particular to NMR spectroscopy and MRI methods and systems for improving the dynamic range of the received signal by using RF excitation pulses with a quadratic (or substantially quadratic) phase in a direction perpendicular to the plane defined by the slab (i.e., in the slab-select direction).
A first aspect of the invention is a method of obtaining a NMR excited signal from a sample placed in a NMR system. The method includes providing an adiabatic RF excitation pulse capable of producing a sharp slab profile to a select portion of the sample (i.e., a slab), and then detecting the resultant excited signal emanating from the slab. An example RF excitation pulse suitable for use in the present invention has a hyperbolic secant (HS) amplitude modulation with a substantially quadratic phase modulation.
A second aspect of the invention includes the method of the first aspect of the invention, where the acts therein are repeated with different phase-encoding gradients until enough data points are collected to achieve a desired resolution. The excited signal data is then transformed (e.g., by discrete Fourier transform) to obtain either a 3-D MRI image of the slab or a NMR spectrum of the slab.
In a third aspect of the invention, multiple slab images formed as described briefly above are combined to form a multiple-slab 3-D MRI image.
A fourth aspect of the invention is a computer-readable medium having instructions for a computer to carry out the methods of the present invention.
A fifth aspect of the invention is a system for performing 3-D MRI or NMR spectroscopy of a slab of a sample. The apparatus includes a magnet that creates a constant magnetic field within the slab, and gradient coils capable of forming phase-encoding gradient magnetic fields within the slab. One or more RF coils are in operable communication with the slab and are capable of providing to the slab adiabatic RF excitation pulses that form a sharp slab profile and a substantially quadratic phase across the slab. The one or more RF coils also can detect an excited signal from the slab. The phase-encoding gradient magnetic fields are used to localize the excited signal within the slab, thus allowing for excited signal data to be collected across the slab.